"The contribution of Canada to the Imperial war effort in troops, in ships, in aircraft, in food, and in finance has been magnificent.....Hitler and his Nazi gang have sown the wind: let them reap the whirlwind.......When I warned them (the French) that Britain would fight on alone whatever they did their generals told their Prime Minister and his divided Cabinet, 'In three weeks England would have her neck wrung like a chicken.' Some chicken! Some Neck."
Canadian Parliament 30 December 1941

When he says "Some chicken", PMs start laughing, and when he says "Some Neck" they laugh again. I'm not a native English speaker. I miss the the joke of this.

What does "some chicken, some neck" mean in this context and why is it funny?

The reason it is funny is that it is an example of irony. He is agreeing that Britain might be like a chicken that could have its neck wrung by its enemies, but that neck had not yet been wrung and Britain would continue to resist its enemies (with or without the help of allies).

My husband of nearly 39 years was English, and I lived in London for a total of 5 years from 1970-76. 'Neck' is slang for audacity, shortened from 'brass neck'. One's anatomical neck is a very vulnerable part of the body which, if it was brass, would offer significant protection against injury, thereby imparting an audacious courage to say provocative things with impunity. So the context of 'neck' as Churchill used it was equivalent to 'some nerve', to have made such a bold threat/underestimated Britain's capabilities as a foe (reference the Battle of Britain when the RAF/Various other nations volunteers prevailed against Goering's Luftwaffe).

Contrary to JLG’s explanation, Churchill is not agreeing that “Britain” [sic] could have its neck wrung by its enemies, but instead using sarcasm to point out that although England had been likened to a chicken — an incredibly stupid, flightless and helpless bird, the very name of which is the epitome of cowardice — the irony was that England was the exact opposite.

Likewise, he was referring to our neck. Normally being one of the body’s weak points, with the help of the rest of the United Kingdom (and the Commonwealth of course) was effectively so thick that it was unable to be “wrung”.

I do not know what your native language is but clearly the key is the translation of "some chicken! some neck!" If I were translating it into French I would render it as "drôle de poulet....! drôle de cou!" which gives the right idea and just as impoortantly the right emphasis (with the added nuance that "neck" in English also has the sense of "culot" in French as others have pointed out). It is not it should be said enormously witty or enormously funny but much is in the performance and the timing.

German forces arrived in an undefended Paris on 14 June 1940 and their commanders met with French officials who sought an alliance with Germany. Chief among these was Marshal Philippe Pétain who, contrary to the wishes of many Frenchmen, announced he would seek an armistice.

French generals hide in England.

"When I warned them that Britain would fight on alone whatever they did, their generals told their Prime Minister and his divided Cabinet, 'In three weeks England will have her neck wrung like a chicken.' Some chicken! Some neck!"